Dynamic energy storage forsmart grids

Frans Dijkhuizen, Willy Hermansson,Konstantinos Papastergiou and Georgios Demetriades

ABB Corporate Research, Vasteras, Sweden, and

Rolf GrunbaumABB AB, FACTS, Vasteras, Sweden

Abstract

Purpose – This paper presents the world’s first high voltage utility-scale battery energy storagesystem in the multi megawatt range.

Design/methodology/approach – The objectives are achieved by the series connection ofswitching semiconductor devices of the type Insulated Gate Bipolar Transistor (IGBT) and the seriesand parallel connection of Li-ion batteries.

Findings – After tests at ABB laboratories, where its performance to specification was confirmed,a first pilot will be installed in the field, in EDF Energy Networks’ distribution network in theUnited Kingdom during 2010 to demonstrate its capability under a variety of network conditions,including operation with nearby wind generation.

Practical implications – This holds the development of a distributed dc breaker, the diagnostics ofdetecting fault locations as well as fault isolation and the balancing of the batteries.

Originality/value – The paper presents the world’s first high voltage utility-scale battery energystorage system in the multi megawatt range suitable for a number of applications in today’s and futuretransmission and distribution systems.

Keywords High voltage, Electric cells, Energy supply systems, United Kingdom

Paper type Research paper

1. IntroductionModern society depends heavily upon electricity. With deregulation and privatization,electricity is becoming a commodity among others. At the same time, growingconstraints on the building of new lines are appearing as a consequence of increasingfocus on environmental aspects as well as of growing scarcity of land available for thepurpose. This invokes a more efficient use of the existing infrastructure rather thanbuilding new ones while the need for power transmission capacity increases, makingthe power transmission capacity more cost effective in a deregulated market. Flexiblealternating current transmission systems (FACTS) is an enabling technology availableto use the existing lines more effectively (Hingorani and Gyugyi, 1999).

The ever-increasing demand for power, and in the long run the integration ofelectric vehicles, and the need of reducing CO2 emissions at the same time, requiresthe integration of more renewable power and more distributed energy supply into thesystem without compromising the reliability of the supply. A future electric system isneeded that handles those challenges in a sustainable, reliable and economic way.The resulting system will be based on advanced infrastructure and tuned to facilitatethe integration of all involved. The forces now driving the development of smart gridsare as varied as they are influential. Environmental concerns are increasing around the

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COMPEL: The International Journalfor Computation and Mathematics inElectrical and Electronic Engineering

Vol. 31 No. 1, 2012pp. 279-293

q Emerald Group Publishing Limited0332-1649

DOI 10.1108/03321641211184977

globe, driving the development of renewable energy on a larger scale than ever before.The widespread addition of wind, solar and other renewables give rise to additionaloperational challenges due to their intermittent nature. A system that can handle ageneration mix with a high percentage of renewables will become a necessity for thosetechnologies to realize their full potential.

For intermittent power sources like wind and solar, the challenge is to connect andintegrate them without compromising the required reliability at a low reserve capacitylevel. This requires viable solutions of electrical energy storage both for distributed andbulk power applications. This paper presents a dynamic energy storage system whichis based on the Static VAr Compensator (SVC) Lightw (Grunbaum et al., 2001a, b), forutility-scale energy storage. This storage system utilizes Li-ion batteries. Thisdevelopment started around 2000 as a technology-driven project. After benchmarkingof battery technologies Li-ion has been chosen in 2007. This followed by a systemdevelopment in close cooperation with the battery manufacturer Saft and the marketlaunch was early 2010. This paper will give first an introduction to smart grids afterwhich the battery energy storage system will be explained. The first pilot installation atEDF Energy Networks in the UK will be presented followed by the electrical storageapplications.

2. Smart gridsTraditional grids as shown in Figure 1(a) are characterized by centralized powergeneration and uni-directional power flow. Future grids as shown in Figure 1(b) arecharacterized by centralized and distributed generation, intermittent renewable powergeneration, and multi-directional power flow where consumers become also producers.The ever increasing demand for power, and the need of reducing CO2 emissions at thesame time, requires the integration of more renewable power and more distributedenergy supply into the system without compromising the reliability of the supply.A future electric system is needed that handles those challenges in a sustainable, reliableand economic way. The resulting system will be based on advanced infrastructure andtuned to facilitate the integration of all involved.

For intermittent power sources like wind and solar, the challenge is to connect andintegrate them without compromising the required reliability at a low reserve capacitylevel. This requires viable solutions of electrical energy storage both for distributed andbulk power applications. Dynamic energy storage will play a vital role in this field.

3. Battery energy storageThe battery energy storage is an add-on to the existing SVC Light, ABBs voltage sourceconverter (VSC)-based FACTS device (Hingorani and Gyugyi, 1999), designed forhigh-power applications and using series-connected insulated gate bipolar transistors(IGBTs) to adapt to high voltage levels. Figure 2 shows a valve of a typical SVC Lightinstallation with several series-connected IGBT devices.

Therefore, a number of batteries must be connected in series to build up the requiredvoltage level. Figure 3 shows a schematic layout of the dynamic energy storage deviceconsisting of an SVC Light together with a number of series-connected batteries onthe DC-side and an artist’s view of the dynamic energy storage, DynaPeaQw. Thus,the device can both inject and absorb reactive power, as an ordinary SVC Light, andactive power due to the batteries.

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The grid voltage and the VSC current set the apparent power SVSC of the VSC. Note thatthe peak of the active power of the battery does not have to be equal to the apparentpower of the VSC. For instance, the apparent power of the SVC Light can be 50 MVAand the active power of the batteries can be equal to 10 MW.

The core range for DynaPeaQ is 5-50 MW and 5-60 min.A number of plants for considerably high power and voltage, based on the VSC

technology have been built for both industrial (Grunbaum et al., 2001a, b, 2003; Larssonand Ratering-Schnitzler, 2000) and power system applications (Grunbaum et al.,2001a,b; Larsson et al., 2001). Some of the applications that SVC Light performs are:

. reactive power compensation;

. power factor correction;

. dynamic voltage control;

Figure 1.(a) Traditional grid and

(b) smart grid

(a)

(b)

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. dynamic voltage balancing when the loads are unsymmetrical and rapidlyfluctuating;

. active filtering of low-order current harmonics; and

. flicker mitigation of electric arc furnaces

The size of the battery energy storage depends on the application. However, a simplereasoning is to assume that a certain active power Pl is injected into the grid during theload time Tl, thus discharging the battery (Figure 4). The total energy El injected intothe grid becomes equal to Pl times Tl. To recharge the battery, approximately the sameamount of energy must be absorbed from the grid by the battery. When charging thebattery with the power Pc, the charging time becomes equal to Tc so that El ¼ Ec.During a certain time, the battery is in an idle state before the cycle is repeated again.

4. Pilot systemABB and Saft have developed a pilot of a new high-voltage dynamic energy storagebased on high-voltage Saft Li-ion batteries and ABB:s SVC Light to combine dynamicenergy storage with reactive power compensation and dynamic voltage control. Aftertests at ABB laboratories, where its performance to specification was confirmed(Hermansson et al., 2009), a first pilot will be installed in the field in EDF EnergyNetworks’ distribution network in the UK during 2010 to demonstrate its capabilityunder a variety of network conditions, including operation with nearby windgeneration. The reactive power rating of this pilot station is 600 kVAr inductive and

Figure 2.SVC Light semiconductorswitching valves withseries-connected IGBTs

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Figure 3.(a) Single line diagram ofDynaPeaQ, the dynamic

storage device consistingof SVC Light with series

and parallel-connectedbatteries on the DC side

and (b) artist’s view ofDynaPeaQ

Battery strings

~

=

+

-PCC

Q P

L Cdc1

Cdc2

udcuvsc

uac

i

#1 #2 #3 #4 #5 #N

(a)

(b)

Figure 4.Battery load cycle

P1^

P1E1

EcPc

load

charge idle

period time

Notes: During the total cycle time T, thebattery is discharged to the grid with powerPl during time Tl and charged with powerPc during time Tc; remaining time Ti is idletime

Pc^

Tc Ti

T

T1

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capacitive, the active power rating is 200 kW-1 h and 600 kW-10 min, the nominal linevoltage is 11 kV. Some details of the Li-ion battery system are shown in Figure 5.

Table I shows some basic data of the pilot system.Figure 6 shows a photo of the pilot installation in EDF Energy Networks’

distribution network in the UK. It is made to demonstrate the concept.

5. Experimental resultsSome tests were made at ABB’s laboratories to verify the performance of the system.

Figure 7 shows graphs of a discharge of 200 kW for 1 h and Figure 8 shows anexample of 600 kW discharge for 4 min.

In these tests, the system controller is set to keep the power constant, which results ina steadily increasing discharge current when the battery voltage decreases during thedischarge.

Figure 9 shows an example of battery charging from a state-of-charge (SOC) ¼approx. 50 percent, up to fully charged at 100 percent. The charging follows a specialprocedure in order to avoid overvoltage and overcharging of the battery cells and allowcell balancing to achieve a good homogeneity among the cells. At the start, the charging is

Figure 5.Battery unit with 13modules and one controlunit (BMM) (left) andbattery units inside acontainer (right)

13 batterymodules (14 cells)

control unit

Nominal line voltage 11 kVNominal voltage uac 2.22 kVNominal DC voltage ^2.18 kVdcNominal reactive power Svsc 0.6 MVAr contActive power 0.2 MW, 1 hMax. active power 0.6 MW, 10 minNominal current I 250 ANominal frequency f 50 HzPulse number p 27Switching frequency fsw 1,350 HzPhase reactor L 4.18 mHDC capacitors Cdc1, Cdc2 1,253mF

Table I.Basic system dataof pilot system

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made with a maximum current of 40 A (set by the battery management modules (BMMs)),and at a level of SOC . approx. 80 percent the maximum charging current is reduceddown to 20 A (determined by the limit value as received from the BMMs) and finally whenthe total battery voltage reaches 5,840 V, corresponding to 4.01 V/cell, the control shiftsand keeps the battery voltage constant at this level until a full charge at 100 percent isreached. During this period also the BMMs automatically take care of the cell balancing.

6. Storage applicationsMany applications with energy storage could be identified (Eyer and Corey, 2009),which are independent of the grid location such as load shifting, spinning reserve orprimary frequency regulation. Examples of storage applications that are locationdependent are T&D facility deferral, grid angular stability, grid voltage stability,renewable energy management, customer energy management and distribution powerquality (Figure 10). Some applications will now be described in more detail.

6.1 Grid stabilizationFirst, the grid angular stability: this is the mitigation of power oscillations by injectionand absorption of real power at periods of 1-2 s. The difference between the phase angleon the generation and the load side cannot be too large. If the utility cannot quickly(within a few cycles) damp the oscillation the power system can collapse. Energystorage systems can assist the utilities to maintain synchronous operation bydischarging to provide power and charging to absorb power as the system loadconditions change (Johansson et al., 2004).

Second, there is the grid voltage stability: this is the mitigation of a degraded voltageby additional reactive power plus the injection of real power for durations up to 2 s.The need for a short response time is high. Figure 11 shows the voltage at the end of twoparallel transmission lines, having a VSC with and without storage. To damp theoscillation with active power a nearly constant voltage can be achieved after 0.5 s wherethe VSC without storage lasts nearly 2 s.

Figure 6.Photo of the pilot

installation in EDFEnergy Networks’

distribution network in theUK

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Third, there is the active damping of power oscillations: in the case of power oscillationsthe reactive power production/consumption capacity of SVC Light may be utilized todamp oscillations. In addition, the active power discharge of the energy storage may bevaried such as to damp the power oscillations. Figure 12(a) shows a system voltagevariation when the VSC with storage is not active and in Figure 12(b) the oscillations areremoved by the injected current (inner waveform) from the VSC with storage.

Figure 7.Discharge of 200 kW, 1 hfrom SOC ¼ 100 percent

6,000 250

200

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100

Bat

tery

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rent

(A

)/B

atte

ry p

ower

(kW

)

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Discharge 200 kW, 1 h test from SOC = 100%

5,800

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vol

tage

(v)

5,600

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%)

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0

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Time (s)

State-of-charge (SOC) diring discharge of 200k W, 1 htest from SOC = 100%

2,500 3,000 3,500 4,000

0 500 1,000 1,500 2,000

Time (s)

2,500 3,000 3,500 4,000

UBatt (v)IBatt (v)P (kw)

Note: Upper curves: battery voltage, current and power during the discharge andlower curve: SOC

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6.2 Frequency regulationFirst, the primary frequency regulation: this implies the mitigation of load-generationimbalance requiring an instant-spinning reserve (or load) to discharge real power fordurations of say 30 min under contingency operations. Such a storage system has toabsorb or deliver power as it fluctuates. Figure 13 shows the principle of primarycontrol.

Figure 8.Discharge of 600 kW,

4 min from SOC ¼ 100percent

State-of-charge (SOC) diring discharge of 600 kW, 4 mintest from SOC = 100%

UBatt (v)IBatt (A)P (kw)

Discharge 600 kW, 4 min test from SOC = 100%5,900

5,800

5700

5,600

5,500

5,400

5,300

5,200

5,100700 800 900 1,000 1,100 1,200 1,300 1,400 1,500

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tery

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)/B

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)

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C (

%)

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tery

vol

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(v)

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700 800 900 1,000 1,100 1,200 1,300 1,400 1,500

Time (s)

Notes: Upper curves: battery voltage, current and power during the discharge andlower curve: SOC

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Large changes in the load affect the speed of generators at power plants. This deviationis picked up by the primary controllers of all generators to respond in a few seconds.The controllers alter the power delivered until a new balance has been found. As thebalance is reached the system frequency stabilizes and remains at a quasi-steady statebut differs from the set point due to the droop. To regulate the frequency the utilities caninstall storage systems that discharge during rising load or charge when loads fall off.In this way, the storage systems protect the generator from the fluctuation in the loadand prevent subsequent frequency variations (Kirby, 2004).

Figure 9.Charging fromSOC ¼ approx.50-100 percent

State-of-charge (SOC) at charging up to SOC = 100%from approx. 50%

charging up to SOC = 100% from approx. 50%

120

100

80

60

40

20

0

UBatt (v)IBatt (A)P (kw)

Bat

tery

cur

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(A

)/B

atte

ry p

ower

(kW

)

SO

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%)

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0 500 1,000 1,500 2,000 2,500 3,000

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Notes: Upper curves: battery voltage, current and power during the discharge andlower curve: SOC

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Then there is the secondary frequency regulation: the secondary control holds themitigation of load-generation imbalance under contingency operations, where thefunction of the secondary control is to restore the frequency to its set point 50/60 Hz andthe power interchange with adjacent control areas to their programmed scheduledvalues. Thus, to ensure that the full reserve of primary control power is available again.

Third, there is the tertiary frequency regulation which is reserves that are notautomatically delivered when required, but are instead instructed.

Figure 10.Utility storage application

examples

Figure 11.Voltage at the end of

parallel transmission lines

1.06

1.04

1.02

Bus

vol

tage

(pu

)

1

0.980 0.5 1 1.5

time (seconds)

2 2.5 3

Notes: ◊ – no control; o – voltage-sourced converter;∆ – voltage-sourced converter + battery energy storage facility

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6.3 Grid operational supportRegulation control: this service implies the continuously balance of the generation andload under normal conditions. This control regime is a minute-by-minutegeneration-load balance within a control area. It uses online generation, storage, orload that is equipped with automatic generation control and that can change the outputquickly (MW/min) to track the moment-to-moment fluctuations in generation.

Conventional spinning reserve: this yields reserve power for up to 2 h with 10 minnotice. To start-up cold power plants require hours and combustion engines a half-hourto get generators ready to accept the load. Energy storage can help utilities maintainrapid reserve, reduce or eliminate the need for supplemental power and bridge the gap.

6.4 Renewable generation supportFor example, power transmission and distribution systems are facing an increasedamount of wind generation where the power generated varies depending on the locationjust as well as on the available wind resources. This creates new challenges for thestable and reliable operation of the power system, requiring, in many instances, networkreinforcement to maintain network reliability. Energy storage will increasingly play a

Figure 12.Power-oscillation damping

(a)

(b)

Notes: (a) Without voltage-sourced converter + batteryenergy storage facility; (b) with voltage-sourcedconverter + battery energy storage facility

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role in economically managing power networks to meet demand and maintain supplyreliability during these generation and demand fluctuations. Solar power fromphoto-voltaic plants is another coming area in the renewable power field. Here, too,dynamic energy storage will offer benefits and widen the usability of the concept as asource of clean, environmentally attractive energy. This includes time shifting (storeand release of wind energy), renewable capacity firming (store during low demands andrelease during high demands) and the like.

6.5 Vehicle grid integrationAn electric vehicle needs to be connected to the grid every time its battery is depleted(Figure 14). Therefore, an infrastructure is needed in the form of charging poles and orpossibly battery exchange stations. Moreover, it is technically feasible to adapt thedynamic energy storage concept to a future battery exchange station. Charging canoccur during off-peak hours or peak hours. Particularly, during peak hours most of thepower system is demanded as a fast ramp-up of power and a higher peak load. Batteryenergy storage could advance the integration particularly in areas with a high numberof electrical vehicles.

Figure 14.Electric vehicle integration

Figure 13.Principle of primaryfrequency regulation

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7. ConclusionThe smart grid is open for all types and sizes of generation and takes care of the demandside and operation in an efficient and reliable way. To balance production sources orsupport the integrity of the power system itself energy storage is a vital aspect toguarantee power quality and bridging power applications. A dynamic storage devicebased on batteries, for utility scale, has been outlined, as well as some of its applications.It is an add-on to the existing, proven high-performance SVC Light, which results inincreased functionality with minimum design modifications. It is able to control bothactive and reactive power with fast response and independently of each other, making itsuitable for a number of applications in today’s and future transmission anddistribution systems.

References

Eyer, J. and Corey, J. (2009), “Energy storage for the electricity grid: benefits and marketpotential assessment guide”, SANDIA Report, SANDIA National Laboratories,Albuquerque, NM.

Grunbaum, R., Gustafsson, T. and Olsson, U. (2001a), “SVC light: evaluation of firstinstallation at Hagfors, Sweden”, paper presented at Cigre Conference, Paris, paper13/14/36-03.

Grunbaum, R., Hasler, J.-P. and Thorvaldsson, B. (2001b), “FACTS: powerful means for dynamicload balancing and voltage support of AC traction feeders”, paper presented at IEEEPower Tech, Porto.

Grunbaum, R., Gustafsson, T., Hasler, J.-P., Larsson, T. and Lahtinen, M. (2003), “STATCOM, aprerequisite for a melt shop expansion – performance experiences”, paper presented atIEEE Power Tech, Bologna.

Hermansson, W., Svensson, J.R., Callavik, M. and Nygren, B. (2009), “Dynamic energy storageusing SVC light and Li-ion batteries”, paper presented at NWPC, Bornholm.

Hingorani, N.G. and Gyugyi, L. (1999), Understanding FACTS: Concepts and Technology ofFlexible AC Transmission Systems, IEEE Press, New York, NY.

Johansson, S.G., Asplund, G., Jansson, E. and Rudervall, R. (2004), “Power system stabilitybenefits with VSC DC-transmission systems”, paper presented at Cigre Conference, Paris.

Kirby, B.J. (2004), Frequency Regulation Basics and Trends, National Laboratory, Oak Ridge, TN,ORNL/TM-2004/291.

Larsson, T. and Ratering-Schnitzler, B. (2000), “SVC light: a utility’s aid to restructuring its grid”,paper presented at IEEE PES Winter Meeting, Singapore.

Larsson, T., Petersson, A., Edris, A., Kidd, D. and Aboytes, F. (2001), “Eagle pass back-to-backtie: a dual purpose application of voltage source converter technology”, paper presented atIEEE PES Summer Meeting, Vancouver.

About the authors

Frans Dijkhuizen received his Electrical Engineering degree in 1995 and the PhDdegree in Electrical Engineering in 2003, both from the Eindhoven University ofTechnology TU/e, the Netherlands. He joined ABB Corporate Research inVasteras in 2001 working in the field of high voltage power electronics for HVDCand FACTS applications. His interest includes the whole area of powerelectronics. Frans Dijkhuizen is the corresponding author and can be contacted at:Frans.dijkhuizen@se.abb.com

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Willy Hermansson, born 1949, received his MSc in Applied Physics fromChalmers University of Technology in 1973. He joined ASEA/ABB in 1974 andhas worked with various development projects such as fracture mechanics andfatigue, zinc oxide surge arresters, ultra sonic techniques and powersemiconductor development (thyristors, GTOs, MCTs, IGBTs and siliconcarbide). His present fields of interest are power devices and topologies for powersystems (HVDC and FACTS) and advanced battery systems for use in energystorage systems.

Konstantinos Papastergiou (SM’98,M’06) obtained a BEng from the TechnologyInstitute of Crete in Greece and a PhD degree from the University of Edinburghunder industrial sponsorship by BAE Systems. He worked on the More-ElectricAircrafts (MOET) EU project as a PostDoctoral Research Fellow with PEMCgroup at the University of Nottingham. In 2006 and 2007 Konstantinos was theco-founder of two technology start-ups in the UK. His research interests arefocused on contact-less transfer of energy, magnetics design and optimisation,and multilevel power electronic converters. Konstantinos Papastergiou has

received a IEEE/IES student award as well as grants from Marie Curie, the UK Royal Society andthe Greek State foundation for Scholarships.

Georgios Demetriades was born in Famagusta, Cyprus. He received his MScdegree in Electrical Engineering at the Democritus University of Thrace inGreece, his Licentiate degree and his PhD degree both in Power Electronics fromthe Royal Institute of Technology, KTH, in Stockholm, Sweden. He worked inCyprus for two years and in 1995 he joined what is today known as ALSTOMPower Environmental Systems in Sweden. In the year 2000 he joined ABBCorporate Research where he is currently working as a research anddevelopment manager. His main research interests include power electronics,

VSC HVDC, FACTS devices, high-frequency DC-DC power resonant converters and highfrequency electromagnetic modelling.

Rolf Grunbaum received his MSc degree in Electrical Engineering from ChalmersUniversity of Technology in Gothenburg, Sweden. He is currently working forABB AB within its FACTS Division, where he is Senior Marketing Manager ofFACTS and Reactive Power Compensation Systems. Mr Grunbaum has beenactive in ABB and previously in Asea for a number of years. Before that, he wasemployed by DISA Elektronik in Skovlunde, Denmark, where he was involved inthe marketing of scientific equipment for fluid flow research. He has also heldpositions as Scientific Counsellor in the Swedish Foreign Service. Mr Grunbaumis a member of Cigre and IEEE and the author of numerous FACTS.

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